Patent Application
20080023265
The invention relates to a silencer with an outer shell which is provided with one or more inlet conduits for leading a gas into the silencer and with one or more outlet conduits for leading the gas from the silencer. The invention also relates to such a specific silencer designed so as to provide means intended for acting as a spark-arrestor. Furthermore, the invention relates to a combustion engine provided with a silencer according to the invention.
Reactive silencers for gas flows comprise one or more through-flowed chambers. It is known in the art of silencer design to supplement such silencers in various ways to improve a generally broad-banded noise reduction spectrum at various frequencies. Such improvement is, e.g., warranted because the un-silenced noise spectrum exhibits a peak at one or more frequencies, or because the silencer would otherwise exhibit dips in the attenuation spectrum, typically because of harmful resonance waves set up in chambers or passages of the silencer.
A general feature of such supplementary methods relies on the use of various types of elements which are essentially not through-flowed.
In the following, the expression “terminated cavity” will be used in the meaning of a cavity which is not through-flowed.
One such method of supplementing a silencer consists of fitting elements containing a sound absorptive material into the silencer, such as mineral wool. Such elements will supplement silencing in a wide frequency range roughly above a certain lower limit frequency, which can be determined by analyzing standing waves set up within cavities containing the absorptive material.
A further aspect of methods of supplementary silencing relies on the use of various types of resonators providing added noise attenuation at one or more selected frequencies. One type of such resonators is the Helmholtz resonator, which is connected to a through-flowed passage or chamber via a neck section, which is in turn connected to a terminated cavity. The peak attenuation frequency of such a Helmholtz resonator may be approximately calculated by a mass-spring analogy. The mass-spring analogy considers the mass of gas present by the neck as a stiff, concentrated mass. The flexibility of the terminated cavity and the flexibility of a cavity on the opposite side of the neck are considered as springs, each spring being connected at one end to the mass and at the other end to a steady wall. If the last-mentioned cavity is much bigger than the terminated cavity, it will act as a relatively much softer spring whose stiffness may be omitted in a rough calculation of the peak attenuation frequency.
A third supplementary silencing element is the side-branch resonator with a closed-end pipe in which standing waves are set up to absorb sound at corresponding noise frequencies. Such a resonator will provide added noise reduction at frequencies corresponding to 0.25, 0.75, 1.25, etc. waves set up in the pipe. The frequency corresponding to 0.25 wave length will be the lowest (and usually the most prominent) one.
It is further known to design silencers in various ways so as to act as spark arrestors, i.e. to prevent any significant amount of glowing particles (sparks) from passing the silencer, thereby reducing the risk of causing harm to human beings or causing fire or explosion in case of any inflammable substances being situated close to the exhaust.
It is easy to fit a screen or some other particle-obstructing means into a silencer, but commonly known means of this kind generally significantly augment the pressure drop across the silencer. There is a need in the market for a silencer which can produce a significant, not necessarily a maximum, spark arresting effect with a low or a very low additional pressure drop across the silencer.
Although many silencers of known designs, even though not having been designed for such a purpose, will in fact have some spark-arresting effect, a gas flow containing particles, such as exhaust from a diesel engine, may gradually compromise a silencer in its acoustic function. One important reason for this is that perforations in the walls of such a silencer, allowing noise to be transmitted into sound absorptive material, may in the course of time become clogged, causing the desired sound absorptive effect to become gradually obstructed.
It may be an object of the invention to provide a silencer of the reactive type with a resonator and having increased noise attenuation within confined dimensional restraints. It may also be an object of the invention to provide a silencer which is based on already applied techniques and which may be redesigned easily and cheaply.
These objects and possible other objects are obtained by a silencer with
The invention combines, in a basically reactive type silencer, the above-mentioned absorptive type of element with either a Helmholtz resonator or with a side-branch resonator in a way which is particularly compact and cheap to manufacture. In several preferred embodiments of the invention, an annular cavity which surrounds the absorptive element is utilised as a resonator to absorb energy at one or more selected noise frequencies. By means of further, simple design features, these noise frequencies can be selected rather freely, as will be shown and explained below.
In order for the at least one through-flowed chamber to truly act to reflect sound waves at the inlet passages to and the outlet passages from the chamber, and for the chamber to function to reduce noise efficiently from a not too high lower cut-off frequency, it is desirable that the cross-sectional area ratios of the passages in respect to the chamber cross-sectional area, and the outlet passages in respect to the chamber are not too big. It is also desirable that the chamber is of a not too small volume. These objects may be obtained by said at least one through-flowed chamber provided with one or more inlet passages for leading gas to said chamber, and being provided with one or more outlet passages for leading gas from said chamber, and where the sum ain of acoustically representative cross-sectional areas of said inlet passages is fulfilling the condition ain<A/3C, and/or where the sum aout of acoustically representative cross-sectional areas of said outlet passages is fulfilling the condition aout<A/3C, and/or where the volume V of said chamber is fulfilling the condition V>6C(√{square root over (((ain+aout)/2)))}3, A being an acoustically representative cross-sectional area of said chamber and C being a constant at least taking the value C=1. In alternative embodiments, the constant C may take the value C=2 or C=3.
According to one possible embodiment of the invention, said at least one terminated cavity is of a longitudinal shape along a longitudinal extension of the silencer, said cavity thereby acting as a resonator attenuating noise at frequencies corresponding to standing waves in the longitudinal direction of the cavity, the lowest of said frequencies corresponding to a standing quarter-wave. This feature presents the advantage that the silencer is tuned to specific frequencies, thus obtaining maximum noise attenuation in the number of terminated cavities. Furthermore, any one and selected wavelength of the frequency may be used for designing the length of the terminated cavity, but still making it possible to maintain the noise-attenuating properties of the remaining elements of the silencer, and not necessarily demanding a re-design of these elements because of a chosen length of the terminated cavity. Thus, the individual design features may be selected and designed rather freely and not necessarily dependent on possible mutual relationships.
As will be demonstrated, such cavities of a longitudinal shape can be accommodated in remarkably simple and compact designs, making the silencer cheap to manufacture. Furthermore, the silencer is well-suited for augmenting the noise reduction effect of the silencer at one or more particular noise frequencies in situations where noise reduction would be insufficient if one were to rely solely on noise reduction caused by noise-reducing effects caused by a combination of the reactive/reflective effect of through-flowed chambers and the noise absorptive effect achieved by means of sound absorptive material.
According to another possible embodiment of the invention, the terminated cavity communicates acoustically with the at least one through-flowed chamber, either directly or via a neck section provided between the at least one terminated cavity and the chamber Intended for being through-flowed by gas. This feature has the general advantage of achieving additional noise-reducing effects at one or more target frequencies by simple designs. In the case of a silencer with several through-flowed chambers, it will for instance be possible to design the noise reduction characteristic of the silencer within wide limits, without resorting to complicated and costly designs.
Depending on the specific design of the silencer, on the possible dimensional restraints and on the intended and/or demanded noise-attenuating properties, the acoustically effective length, L, of said terminated cavity is essentially equal to, smaller than, or longer than a length of said protruding sound absorptive material seen along the longitudinal extension of the silencer.
When a terminated cavity is used as a side-branch resonator, it should be of a longitudinal shape in order to achieve a distinct resonance effect in the longitudinal direction. This may be achieved by ensuring that a mean distance between walls in radial direction, integrated over the entire longitudinal and circumferential extension of the acoustically effective length of the terminated cavity, is at the most a ratio 1/3 of said acoustically effective length. Alternatively, the mean distance may be at the most a ratio 1/5 of said acoustically effective length. As a further alternative, the mean distance may be at the most a ratio 1/10 of said acoustically effective length.
When a terminated cavity is of a more complex shape, the longitudinality of the cavity can be quantified by referring to a volume V of the cavity and to summed surfaces S of side walls of the cavity. In such a case, the acoustically effective length L of said at least one terminated cavity is at least 6 times the ratio V/S between the volume of said cavity and the summed surface areas of the side walls of said terminated cavity. Alternatively, the acoustically effective length L of said at least one terminated cavity is at least 10 times the ratio V/S. As a further alternative, said acoustically effective length L of said at least one terminated cavity is at least 20 times said ratio V/S.
The shape of said cavity may at least partly constitute a helical winding around said protruding sound absorptive material. A helical winding entails the advantage that the acoustically effective length may be longer than a length of said protruding sound absorptive material seen along the longitudinal extension of the silencer, however, without taking up as much dimensional space seen in either an axial or a radial direction in comparison with the actual acoustically effective length obtained by a helical winding.
If the silencer is provided with at the least one neck section acoustically connecting said at least one terminated cavity with said at least one through-flowed chamber, the neck section may be designed for creating a Helmholtz-type resonator. Thereby it is possible to obtain a very pronounced additional noise reduction effect at a frequency which can be selected within a very broad range of frequencies. For instance, it becomes possible to extend the noise reduction spectrum down to very low frequencies, which may otherwise be difficult to attenuate with a silencer to be accommodated within a limit space restriction, i.e. if the silencer has to be compact.
Furthermore, the at least one protruding sound absorptive material and the at least one essentially surrounding terminated cavity may be of a substantially circular-cylindrical configuration. This is an example as to how the invention makes it possible to accommodate three, mutually supplementing noise reduction effects in a particularly simple design. Those three effects are, noise reduction by: 1) sound reflection at cross-sectional changes at inlets and outlets of through-flowed chambers, 2) sound absorption in sound absorptive material, and 3) resonance sound absorption, either by a longish quarter-wave sound absorber, or by a Helmholtz sound absorber, all those types having been described above.
A possible embodiment of a silencer comprises at least two terminated cavities, the one terminated cavity having an acoustically effective length L1 and the at least one other terminated cavity having an acoustically effective length L2, and where said length L1 is different form said length L2. This is a demonstration of a rather simple way of designing the noise reduction characteristics of a silencer according to the invention. If more than two lengths are selected, a more sophisticated design of the noise reduction characteristics can be attained. One may target very different frequencies in some cases. In other cases, two or more targeted frequencies may differ only slightly, whereby the rather narrow-banded sound absorptive effect of a single resonator can be extended in terms of frequency range effect of the sound absorptive effect.
In order to design the outer shape of a big silencer, for instance the silencer of a big diesel engine serving the electricity needs of an island, in which the allowable length of the silencer is limited, but where it is possible design for a certain width and a certain height of a horizontally disposed silencer, at least three pieces of said protruding sound absorptive material are arranged to establish at least three parallel gas flows inside said protruding sound absorptive material, and the at least one terminated cavity is constituted by a spacing between said protruding sound absorptive material.
This cavity may be a residual cavity found between the three or more protrusions. The cross-sectional shape of this residual cavity may appear complicated, but since it is a residual cavity, it can be accommodated by a very simple design, as will be demonstrated below. If the residual cavity is used as a quarter-wave resonator, it should be of a rather longish shape, which can be obtained simply by designing the protrusions to be of a certain minimum length and by accommodating the protrusions close to each other, so that the mean transverse direction between the protrusions becomes rather small.
If at least three protruding pieces of sound absorptive material are provided, more of said cavities are formed by sub-dividing said spacing, possibly by sub-dividing said spacing by establishing insertions of one or more walls between said protruding sound absorptive material. The sub-division of the spacing may be established by inserting one or more walls between the protruding sound absorptive material and another member of the silencer, e.g. the outer shell of the silencer. The sub-division can be used for having different lengths of different quarter-wave resonators. Walls for subdivision may nevertheless be necessary to provide sufficient stiffness to the assembly of protrusions.
At least one resonator of the silencer is constituted by at least one terminated cavity at least partly surrounding the protruding sound absorptive material, tuned so as to target a peak frequency of an un-attenuated noise spectrum to be attenuated by the silencer.
According to an aspect of the invention, the silencer is designed so as to provide means intended for acting as a spark-arrestor, said means comprising the at least one terminated cavity capable of collecting particles separated from the gas flow as the spark-arresting function. Preferably, at least one of the following flow motions; radial outward flow motion, radial inward flow motion and swirling flow motion, is enforced upon the gas flow passing through the silencer, thereby promoting particles contained in the gas flow to be collected in the at least one terminated cavity designed as a spark-arrestor.
Spark-arresting function is commonly called for in exhaust systems, especially exhaust systems of diesel engines, where the exhaust will contain particles of various sizes. Such particles will tend to clog any perforations in communication with the sound absorptive material, especially in cases where the perforations are not continuously being swept by a gas flow. Such clogging can gradually compromise the sound absorptive function of the sound absorptive material. Larger particles may be glowing for a long time after having been exhausted form the engine as such, and if such glowing particles are transmitted to the environment, they may cause harm such as fires or even explosions. Thus, according to another aspect, the invention also relates to a combustion engine provided with a silencer according to the invention, said combustion engine preferably being a diesel engine.
It will be demonstrated below how the invention makes it possible to attain a significant spark-arresting effect without resorting to design elements that would cause a significant increase of pressure drop across the silencer.
It will also be demonstrated how the invention can be adapted to contain instead one or more screens inside a silencer in a way that provides an enhanced spark arresting function in such a way that there will be less accumulation of soot or other particles onto such screens, compared to prior art silencers providing spark arresting by means of screens.
In the following, the invention will be described in more detail by reference to the figures where:
a shows a longitudinal cross-section of a third embodiment of the invention,
b shows a transverse cross-section of the third embodiment of the invention,
a shows a longitudinal cross-section of a sixth embodiment of the invention,
b shows a transverse cross-section of the sixth embodiment of the invention,
a and 7b show cross-sections of a seventh embodiment of the invention and
a and 8b show cross-sections of an eighth embodiment of the invention.
From a position 14 in the immediate vicinity of the end cap 5, alternatively at a position more or less remote from the end cap 5, the perforated pipe section 7 is surrounded by sound absorptive material 13 contained between the pipe and a cylindrical shell 15. The whole assembly of pipe section 7, sound absorptive material 13, cylinder 4 and radial diffuser 8 all together provide a protrusion 16 from the end cap 5 at the first pipe 1 into the silencer interior. The protrusion 16 may also be constituted only by at least the pipe section 7 and the sound absorptive material 13, thus omitting either the cylinder 4 or the radial diffuser 8, or omitting both the cylinder 4 and the radial diffuser 8. The protrusion 16 is surrounded by an annular spacing, providing a terminated cavity 17 between the cylindrical shell 15 and the outer casing 3 of the silencer.
The terminated cavity 17 acts as a side-branch resonator acoustically communicating with the through-flowed chamber 12 through an opening 18. Inside the resonator, during operation of the silencer, standing waves are set up, said standing waves having the effect of absorbing sound energy from the through-flowed chamber 12 at corresponding frequencies. The lowest of these frequencies is the one whose quarter-wave pressure amplitude p variation from the closed end of the terminated cavity 17 to the opening 18 is shown adjacent to the silencer. In the embodiment shown, the resonator length L1 is essentially the same as the length P1 of the protrusion 16.
The second pipe 2 extends backwards into the silencer by a perforated pipe section 27 which is surrounded by sound absorptive material 23 extending from a position 24 in the immediate vicinity of the end cap 6, alternatively at a position more or less remote from the end cap 6. The sound absorptive material 23 is surrounded by a cylindrical shell 25. Smooth gas flow from the through-flowed chamber 12 into the second pipe 2 is provided for by means of a conical baffle 19, which together with the cylindrical shell 25 acoustically closes off the sound absorptive material 23 against the through-flowed chamber 12. Between the cylindrical shell 25 and the cylindrical silencer casing 4, a terminated cavity 27 is formed, which is an annularly shaped side-branch, communicating acoustically with the through-flowed chamber 12 via an opening 28.
The perforated pipe section 27, the sound absorptive material 23, the cylindrical shell 25, and a conical baffle 19 together constitute a backward protrusion 26 into the silencer. The length L2 of the terminated cavity 27 has been made shorter than the length P2 of the backward protrusion 26 by insertion of an annular baffle 29 between the cylindrical shell 25 and the cylindrical casing 4 of the silencer.
This feature has the effect of shortening the length of the quarter-wave, as can be seen from the small diagram of pressure amplitude, shown adjacent to the silencer. Corresponding to the shortened quarter-wave is a higher peak frequency of sound absorption associated with the resonator, for which it could be desirable to target a certain peak in the un-attenuated noise frequency spectrum, or a dip in the attenuation spectrum of a corresponding silencer without the annular baffle 29.
Summing up,
a and 3b show a third embodiment of the invention. In addition to noise reduction, this silencer has been designed to act as a spark-arrestor in which particles 31 are collected at the bottom of an annular, terminated cavity 17. The particles accumulating at the bottom of the terminated cavity 17 can be sucked out of the cavity 17 via a suction pipe 32. A radial diffuser 8 has been fitted with bending ribs 11 as can be seen from
Thereby, especially larger particles, e.g. those whose glow is potentially the most harmful to the environment if not retained, are forced towards the inner side of the silencer casing, from which the particles, due to gravity, will fall downwards to be collected at the bottom of the cavity as indicated by dotted arrows in
It is to be noted that the radial diffuser 8, as indicated in
For a given distance between baffles 9 and 10 in the embodiment shown in
In
It should be noted that, although as mentioned, the flow through the silencer has been forced to follow rather abrupt changes of flow direction, there will not be any big pressure drop across the silencer. One important reason for this can be found in the use of the radial diffuser, which can be designed for a negative pressure (i.e. a pressure recovery) across this particular design element. Also, collecting the particles in the second cavity 27 is associated with a minimal pressure loss, since the flow, when forced inwardly from the outer shell has a rather low flow velocity.
The second feature illustrated in
This variation of length L increases the band-width of attenuation, caused by the side-branch, at the expense of a lowered peak attenuation. Such increase of band-width may be desirable if the frequency in which added attenuation is needed is not known exactly and/or if it varies, e.g. due to varying speed of an engine.
a and 6b show a sixth embodiment of the invention, being a silencer with a first and a second through-flowed chamber 12 and 22, separated by a baffle wall 40. Gas flow is led into a first through-flowed chamber 12 via a first, conical diffuser 41. The connection between the two chambers is made of in total twenty small absorptive silencers, each providing a protrusion from the baffle wall towards a second through-flowed chamber 22. Each small silencer is made of a perforated pipe 42 ending in a small, conical diffuser 43. Between the protrusions and the inner side walls of the silencer casing 4 a residual spacing is created, constituting a terminated cavity assembly 17, which acts as a collective side-branch resonator, as illustrated by the quarter-wave pressure diagram in
In
Quarter-waves are most prominently set up in side-branch resonators when such resonators are pipes or function as pipes by being of a longitudinal, rather narrow shape, to create a rather distinct resonance in the longitudinal direction. Thus, in the embodiments shown in
In all previously shown embodiments, the perforated pipes and the protruding sound absorptive material have been shown to be circular-cylindrical, and the sound absorptive material as well as the terminated cavities have been shown to surround the perforated pipes.
In all the embodiments shown, the silencers are shown to be designed with a simple outer casing. It should be understood that acoustical and spark-arresting effects similar to those demonstrated in the embodiments may also be achieved with more complicated designed silencer casings. For the sake of heat insulation, it may e.g. be preferable to design the casing as a double-wall design or multiple-wall design, possibly with heat-insulating material in spacings between the walls.
Further, in all the embodiments, sound absorptive material 13,23 is shown to surround perforations 7,27 of pipes in immediate vicinity of the pipes. In order to minimize the amount of sound absorptive material drawn out into the pipe at large gas velocities, protective open-structured (not preventing sound propagation) intermediate layers may separate the perforations 7,27 and the sound absorptive material 13,23.
a and 8b show cross-sections of an eighth embodiment of the invention. Here, a per se known type of screen 47 has been fitted onto the cylinder 25 in such a way that all gas flowing through the silencer is forced to pass the screen 47. The screen 47 may e.g. be provided by a perforated plate or by one or more woven, metallic thread networks. The sizes of the openings in the screen 47 will be chosen so that they will not allow any particles greater than a certain size, for instance 0.5 mm, to pass through the screen.
Flow enters the silencer tangentially to the cylindrical shell 4 via inlet pipe 1 so that a rotating flow pattern is set up inside the chamber of the silencer to the left of the assembly 26 extending backwards from silencer outlet Into the interior of the silencer. As a result of centrifugal forces acting on particles, this rotational flow will cause bigger particles to move towards the inner surface of the shell, so that such bigger particles, following paths indicated by to dotted curves, will enter the terminated cavity 27 via the annular opening 28, to accumulate within this cavity.
Smaller particles will be subject to less strong centrifugal forces and will therefore be more prone to follow the gas stream which will converge towards the inflow 2 via the screen. Any particle that is sufficiently small not to be captured in the terminated cavity, but which is bigger than the size of the openings of the screen, will be retained on the upstream side of the screen. The left-hand end cap 5 of the silencer is removable (as has been indicated by flanges and bolts), which permits access to the screen for cleaning it from the outside.
Compared to a prior art silencer providing spark-arrestor function by using a screen, the eighth embodiment of the invention has the advantage that, since a significant part of soot is retained within the terminated cavity, there will be less build-up of soot on the screen, causing less increase of pressure drop and/or permitting less frequent cleaning of the screen.
Although all the previous embodiments of the invention shown in FIGS. 1 to 7, when properly designed, may in many cases provide a satisfactory spark arresting function, insertion of a screen can be motivated to improve the spark-arresting function in various circumstances. Conversely, the eighth embodiment of the Invention can be preferable to other types of silencers comprising a screen, due to better acoustical performance, as well as further considerations. The screen shown in
By using small openings of the screen even very small particles can be retained. When spark-arresting relies on a screen only, the designer will tend to not choose too small openings, since that could result in rather rapid build-up of soot on the screen. When the silencer is designed to be relatively small, particles following the gas flow will reside inside the silencer only during a short time interval. This may call for use of a screen with small openings. Another example in which adding a screen can be motivated is when an engine sometimes runs at low rotational speed, in which case gas velocities within the silencer will be small, causing a decrease of centrifugal forces on particles.
A further aspect is that some regulations specifically demand that spark-arresting effect be attained by use of screens with a specific geometry. In such cases, the formal demand can be fulfilled without heavy build-up of soot or other particles onto the screen, by using the last shown embodiment of the invention. A still further aspect is that in the eighth embodiment of the invention, even if a screen should happen to break down mechanically, the silencer will retain its significant spark-arresting function; i.e. the double spark-arresting function represents a non-trivial redundancy.
The drawing shows a preferred embodiment in which a screen has been arranged inside a silencer according to the invention. Since that screen a diameter which is much bigger than both pipes 1,2, the pressure drop across the screen will be relatively small. The effective area of the screen can be further increased by shaping it in various ways apart from a plane shape. Thus, for instance, the screen can be made dome-shaped, which adds the advantage of a greater mechanical stability of the screen.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2004 00851 | May 2004 | DK | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DK05/00353 | 5/27/2005 | WO | 8/6/2007 |
